B. Van Compernolle

The many faces of shear Alfvén wavesa)

One of the fundamental waves in magnetized plasmas is the shear Alfven wave. This wave is responsible for rearranging current systems and, in fact all low frequency currents in magnetized plasmas are shear waves. It has become apparent that Alfven waves are important in a wide variety of physical environments. Shear waves of various forms have been a topic of experimental research for more than fifteen years in the large plasma device (LAPD) at UCLA. The waves were first studied in both the kinetic and inertial regimes when excited by fluctuating currents with transverse dimension on the order of the collisionless skin depth. Theory and experiment on wave propagation in these regimes is presented, and the morphology of the wave is illustrated to be dependent on the generation mechanism. Three-dimensional currents associated with the waves have been mapped. The ion motion, which closes the current across the magnetic field, has been studied using laser induced fluorescence. The wave propagation in inhomogeneous magnetic fields and density gradients is presented as well as effects of collisions and reflections from boundaries. Reflections may result in Alfvenic field line resonances and in the right conditions maser action. The waves occur spontaneously on temperature and density gradients as hybrids with drift waves. These have been seen to affect cross-field heat and plasma transport. Although the waves are easily launched with antennas, they may also be generated by secondary processes, such as Cherenkov radiation. This is the case when intense shear Alfven waves in a background magnetoplasma are produced by an exploding laser-produced plasma. Time varying magnetic flux ropes can be considered to be low frequency shear waves. Studies of the interaction of multiple ropes and the link between magnetic field line reconnection and rope dynamics are revealed. This manuscript gives us an overview of the major results from these experiments and provides a modern prospective for the earlier studies of shear Alfven waves.

The upgraded Large Plasma Device, a machine for studying frontier basic plasma physics

In 1991 a manuscript describing an instrument for studying magnetized plasmas was published in this journal. The Large Plasma Device (LAPD) was upgraded in 2001 and has become a national user facility for the study of basic plasma physics. The upgrade as well as diagnostics introduced since then has significantly changed the capabilities of the device. All references to the machine still quote the original RSI paper, which at this time is not appropriate. In this work, the properties of the updated LAPD are presented. The strategy of the machine construction, the available diagnostics, the parameters available for experiments, as well as illustrations of several experiments are presented here.

THREE-DIMENSIONAL RECONNECTION INVOLVING MAGNETIC FLUX ROPES

Two and three magnetic flux ropes are created and studied in a well-diagnosed laboratory experiment. The twisted helical bundles of field lines rotate and collide with each other over time. In the two rope case, reverse current layers indicative of reconnection are observed. Using a high spatial and temporal resolution three-dimensional volume data set in both cases, quasi-separatrix layers (QSLs) are identified in the magnetic field. Originally developed in the context of solar magnetic reconnection, QSLs are thought to be preferred sites for reconnection. This is verified in these studies. In the case of three flux ropes there are multiple QSLs, which come and go in time. The divergence of the field lines within the QSLs and the field line motion is presented. In all cases, it is observed that the reconnection is patchy in space and bursty in time. Although it occurs at localized positions it is the result of the nonlocal behavior of the flux ropes.

Observation of collisionless shocks in a large current‐free laboratory plasma

We report the first measurements of the formation and structure of a magnetized collisionless shock by a laser-driven magnetic piston in a current-free laboratory plasma. This new class of experiments combines a high-energy laser system and a large magnetized plasma to transfer energy from a laser plasma plume to the ambient ions through collisionless coupling, until a self-sustained MA∼ 2 magnetosonic shock separates from the piston. The ambient plasma is highly magnetized, current free, and large enough (17 m × 0.6 m) to support Alfven waves. Magnetic field measurements of the structure and evolution of the shock are consistent with two-dimensional hybrid simulations, which show Larmor coupling between the debris and ambient ions and the presence of reflected ions, which provide the dissipation. The measured shock formation time confirms predictions from computational work.

Generation of suprathermal electrons and Alfvén waves by a high power pulse at the electron plasma frequency

The interaction of a short high power pulse at the electron plasma frequency (f=9GHz, pulse length τ=0.5μs or 2.5μs, input power P<80kW) and a magnetized plasma (n0⩽2×1012cm−3, B0=1–2.5kG, helium) capable of supporting Alfven waves has been studied. The interaction leads to the generation of field aligned suprathermal electrons and shear Alfven waves. The experiment was performed both in ordinary mode (O mode) and extraordinary mode (X mode), for different background magnetic fields B0 and different power levels of the incoming microwaves.

Morphology and dynamics of three interacting kink-unstable flux ropes in a laboratory magnetoplasma

Flux ropes are ubiquitous in space and solar plasmas. Multiple adjacent flux ropes are commonly observed both in the solar corona and in the earths magnetotail. The interaction of adjacent flux ropes is often dynamic and can lead to magnetic reconnection. In this paper, the interaction of three flux ropes is studied in a low β background laboratory magnetoplasma. The magnetic structure of the flux rope is produced by the poloidal field of a field-aligned finite sized current which adds to the guide magnetic field and creates the typical helical field line structure. Each rope produces magnetic fields on the order of a few percent of the guide field. Volumetric magnetic field data were acquired and the magnetic field structure and dynamics of the flux ropes can thus be reconstructed. The flux ropes are found to propagate at the Alfven speed. Merging and bouncing of the flux ropes have been observed. The ropes twist and writhe as they propagate through the plasma. They are line tied and clearly separate at...

Resonant excitation of whistler waves by a helical electron beam

Geophysical Research Letters RESEARCH LETTER 10.1002/2015GL067126 Key Points: • Chorus-like whistler mode waves are excited in a laboratory plasma to study excitation process • The resonance structure of whistler wave is experimentally resolved for the first time • Linear theory shows consistent behavior in both intensity and wave normal angle Supporting Information: • Supporting Information S1 • Movie S1 • Movie S2 • Movie S3 • Movie S4 Correspondence to: X. An, xinan@atmos.ucla.edu Citation: An, X., B. Van Compernolle, J. Bortnik, R. M. Thorne, L. Chen, and W. Li (2016), Resonant excitation of whistler waves by a helical electron beam, Geophys. Res. Lett., 43, 2413–2421, doi:10.1002/2015GL067126. Received 21 NOV 2015 Accepted 21 JAN 2016 Accepted article online 25 JAN 2016 Published online 29 MAR 2016 ©2016. American Geophysical Union. All Rights Reserved. AN ET AL. Resonant excitation of whistler waves by a helical electron beam X. An 1 , B. Van Compernolle 2 , J. Bortnik 1 , R. M. Thorne 1 , L. Chen 3 , and W. Li 1 1 Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California, USA, 2 Department of Physics, University of California, Los Angeles, California, USA, 3 Physics Department, W. B. Hanson Center for Space Sciences, University of Texas at Dallas, Richardson, Texas, USA Abstract Chorus-like whistler mode waves that are known to play a fundamental role in driving radiation belt dynamics are excited on the Large Plasma Device by the injection of a helical electron beam into a cold plasma. The mode structure of the excited whistler wave is identified using a phase correlation technique showing that the waves are excited through a combination of Landau resonance, cyclotron resonance, and anomalous cyclotron resonance. The dominant wave mode excited through cyclotron resonance is quasi-parallel propagating, whereas wave modes excited through Landau resonance and anomalous cyclotron resonance propagate at oblique angles that are close to the resonance cone. An analysis of the linear wave growth rates captures the major observations in the experiment. The results have important implications for the generation process of whistler waves in the Earth’s inner magnetosphere. 1. Introduction Naturally generated whistler mode chorus waves in the Earth’s near-space environment are known to play a key role in the acceleration and loss of the relativistic electrons that comprise the radiation belts [e.g., Thorne, 2010; Thorne et al., 2013]. They appear in various forms, such as discrete rising or falling tones, and broad- band hiss emissions [Pope, 1963; Burtis and Helliwell, 1969; Cornilleau-Wehrlin et al., 1978; Koons, 1981; Santolik et al., 2009; Li et al., 2012]. These chorus waves are commonly separated into two distinct bands with a gap at 0.5Ω e ( Ω e is the electron gyrofrequency) [Tsurutani and Smith, 1974; Hayakawa et al., 1984; Li et al., 2013]. The lower band ( 𝜔∕Ω e 0.5) has a broad range of wave normal angles between 0 ∘ and 60 ∘ [Li et al., 2013]. Several mechanisms have been proposed to account for the banded structure of chorus [Omura et al., 2009; Liu et al., 2011; Fu et al., 2014; Mourenas et al., 2015; Fu et al., 2015]. Among the proposed mechanisms, Mourenas et al. [2015] suggested that the less frequently occurring but statistically significant very oblique lower band chorus waves can be gener- ated through a combination of cyclotron resonance and Landau resonance with low-energy electron beams having an energy of a few keV. But these theoretical ideas remain to be tested by satellite observations and laboratory experiments. In order to study the excitation of chorus-like whistler waves in a laboratory plasma, an electron beam is used as the free energy source, injected into a cold background plasma. In a beam-plasma system, vari- ous instability processes can excite a variety of waves including whistler mode waves [Bell and Buneman, 1964], Bernstein-mode waves [Kusse and Bers, 1970; Mizuno et al., 1971], and Langmuir waves [O’Neil et al., 1971; Gentle and Lohr, 1973]. Whistler mode emissions by beam-plasma interaction have been studied exten- sively in the past, such as in the generation of auroral hiss [Maggs, 1976; Gurnett et al., 1983; Sazhin et al., 1993], in active experiments in the space environment [Lavergnat and Pellat, 1979; Tokar et al., 1984; Gurnett et al., 1986; Farrell et al., 1988; Neubert and Banks, 1992], and in controlled laboratory settings [Stenzel, 1977; Krafft et al., 1994; Starodubtsev and Krafft, 1999; Starodubtsev et al., 1999; Van Compernolle et al., 2015]. During active experiments of the Spacelab 2 mission, for instance, beam-generated whistler mode emissions were observed to propagate near the resonance cone and were attributed to Landau resonance [Gurnett et al., 1986; Farrell et al., 1988]. In controlled laboratory experiments, whistler mode emissions were also gener- ated through both Landau resonance [Krafft et al., 1994] and cyclotron resonance [Starodubtsev and Krafft, 1999] using a density-modulated electron beam. However, if the electron beam is not modulated, WHISTLER WAVE EXCITATION

Laser-driven, magnetized quasi-perpendicular collisionless shocks on the Large Plasma Devicea)

The interaction of a laser-driven super-Alfvenic magnetic piston with a large, preformed magnetized ambient plasma has been studied by utilizing a unique experimental platform that couples the Raptor kJ-class laser system [Niemann et al., J. Instrum. 7, P03010 (2012)] to the Large Plasma Device [Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] at the University of California, Los Angeles. This platform provides experimental conditions of relevance to space and astrophysical magnetic collisionless shocks and, in particular, allows a detailed study of the microphysics of shock formation, including piston-ambient ion collisionless coupling. An overview of the platform and its capabilities is given, and recent experimental results on the coupling of energy between piston and ambient ions and the formation of collisionless shocks are presented and compared to theoretical and computational work. In particular, a magnetosonic pulse consistent with a low-Mach number collisionless shock is observed in a quasi-perpendicular geometry in both experiments and simulations.

Wave and transport studies utilizing dense plasma filaments generated with a lanthanum hexaboride cathode

A portable lanthanum hexaboride (LaB6) cathode has been developed for use in the LArge Plasma Device (LAPD) at UCLA. The LaB6 cathode can be used as a tool for many different studies in experimental plasma physics. To date, the cathode has been used as a source of a plasma with a hot dense core for transport studies and diagnostics development, as a source of gradient driven modes, as a source of shear Alfven waves, and as a source of interacting current channels in reconnection experiments. The LaB6 cathode is capable of higher discharge current densities than the main barium oxide coated LAPD cathode and is therefore able to produce plasmas of higher densities and higher electron temperatures. The 8.25 cm diameter cathode can be introduced into the LAPD at different axial locations without the need to break vacuum. The cathode can be scaled up or down for use as a portable secondary plasma source in other machines.

Pulsating Magnetic Reconnection Driven by Three-Dimensional Flux-Rope Interactions

The dynamics of magnetic reconnection is investigated in a laboratory experiment consisting of two magnetic flux ropes, with currents slightly above the threshold for the kink instability. The evolution features periodic bursts of magnetic reconnection. To diagnose this complex evolution, volumetric three-dimensional data were acquired for both the magnetic and electric fields, allowing key field-line mapping quantities to be directly evaluated for the first time with experimental data. The ropes interact by rotating about each other and periodically bouncing at the kink frequency. During each reconnection event, the formation of a quasiseparatrix layer (QSL) is observed in the magnetic field between the flux ropes. Furthermore, a clear correlation is demonstrated between the quasiseparatrix layer and enhanced values of the quasipotential computed by integrating the parallel electric field along magnetic field lines. These results provide clear evidence that field lines passing through the quasiseparatrix layer are undergoing reconnection and give a direct measure of the nonlinear reconnection rate. The measurements suggest that the parallel electric field within the QSL is supported predominantly by electron pressure; however, resistivity may play a role.